Cyclobutane bond energies

TABLE 8. Bond energies and strain energies of cyclopropane, cyclobutane and propane as calculated by the virial partitioning method"... 

The difference in the values of the CC bond energies for 1 and cyclobutane does not contradict observations made for bond orders and overlap values, namely that... 

Neither bond order or CC overlap values alone can lead to an estimate of the CC bond energy and, in this respect, the observation that both 1 and cyclobutane possess similar CC bonding overlap51 does not say anything with regard to the CC bond strength. 

Cremer, D. Gauss, J. Theoretical determination of molecular structure and conformation. 20. Reevaluation of the strain energies of cyclopropane and cyclobutane - CC and CH bond energies, 1,3 interactions, and o-aromaticity, J. Am. Chem. Soc. 1986, 108, 7467-7477. 

A recently reported investigation of the gas-phase iodination of cyclobutane between 589 and 662 K provides a value of Afff (298) = 51.14( 1.0)kcal mol for cyclobutyl radical. The C—bond energy of cyclobutane was found to be 1.8 kcal mol" higher than that of a normal secondary C—H bond and this was related to the increase in strain upon the development of an sp hybridized carbon in a four-membered ring. In gas-phase chlorinations, cyclobutane is also known to be less reactive than cyclopentane and cyclohexane. However, an interesting result has been obtained in the gas-phase chlorinations of chlorocyclobutane between 35 and 195 °C, and of methyl-cyclobutane between 74 and 150°C at ca. 58Torr. " Both substituted compounds are more reactive than the cyclopentyl and cyclohexyl homologues and this has been ascribed to release of some steric strain with formation of the substituted cyclobutyl radical. This is particularly the case for reaction at the tertiary position. 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

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